This invention relates to fluorocarbons for use as refrigerants and more particularly relates to fluorocarbon 1,1,1,3,3,3-hexafluoropropane (HFC-236fa) and methods for its purification.
Due to their stability, relatively low toxicity, compatibility with numerous substances, and low vaporization points, short chain fluorocarbons have been found to have utility or potential utility in industry for many purposes. Such short chain fluorocarbons (about 1 to about 5 carbon atoms) have, for example, been used as propellants, refrigerants, and solvents.
It has been asserted that certain fluorocarbons, especially chlorofluorocarbons, unfortunately may be hazardous to the environment, especially to the ozone layer. Further, impurities in certain fluorocarbons have been found to be troublesome. Unsaturated fluorocarbons are especially undesirable since many of them are toxic and are also often unstable. Such unstable unsaturated fluorocarbons can decompose into even more undesirable products which can be corrosive. It is therefore especially important that fluorocarbons be essentially free (e.g. less than 500 ppm) of unsaturated species. There has therefore been a concerted effort to develop new and purer fluorocarbons which are viewed as being more environmentally friendly.
With respect to some fluorocarbons, it has been found possible to reduce the quantity of unsaturated species by chlorination, e.g. as described in U.S. Pat. Nos. 5,190,626 and 5,336,377. Unfortunately the outcome of such a chlorination technique is not predictable from one fluorocarbon to another, especially when hydrogen is present in the fluorocarbon being purified. This is because hydrogen is frequently replaced by chlorine which reduces yield of the desired product and results in yet further impurities. In certain applications, such fluorocarbons must be especially pure, e.g. when they are used in particularly sensitive areas such as for refrigerants in air conditioners in confined areas.
It has been found that, 1,1,1,3,3,3-hexafluoropropane (HFC-236fa), can be used as a replacement for the fluorocarbon CCl.sub.2 F.sub.2 CCIF.sub.2 (CFC-114), used as a refrigerant. HFC-236fa can be prepared by numerous methods, e.g. as described in U.S. Pat. Nos. 5,395,997; 5,414,165; and World Patent Application WO96/15085-A1.
1,1,1,3,3,3-hexafluoropropane might be an excellent replacement for CFC-114, except that pure 1,1,1,3,3,3-hexafluoropropane has not been obtainable by known methods of preparation. This due to the fact that methods for the preparation of 1,1,1,3,3,3-hexafluoropropane result in residual impurities of other fluorocarbons. Unfortunately, some of the fluorocarbon impurities form low boiling azeotropes with 1,1,1,3,3,3-hexafluoropropane or are close boiling with 1,1,1,3,3,3-hexafluoropropane, which prevents separation by conventional distillation methods ("distillation inseparable mixtures"). Such distillation inseparable mixtures are obtained when an attempt is made to purify 1,1,1,3,3,3-hexafluoropropane by distillation from the reaction mixture in which it is made. This is especially troublesome since some of the impurities which form azeotropes or close boiling mixtures are unsaturated and cannot be tolerated to any significant extent in refrigerants in certain applications. Examples of such undesirable unsaturated fluorocarbon impurities are 1,1,1,3,3-pentafluoro-2-chloropropene obtained by the liquid phase reaction described in U.S. Pat. No. 5,395,997 and C.sub.3 HCl.sub.2 F.sub.3 obtained by the vapor phase reaction described in U.S. Pat. No. 5,414,165.